This invention relates to a process for purifying aluminum, and more particularly to a process for purifying aluminum containing impurities which form a eutectic with the aluminum to selectively obtain a fraction of higher purity.
Throughout the specification, the term "smooth" refers to the state of a surface which is completely smooth and also to that of a surface having some minute irregularities.
When aluminum containing impurities, such as Fe, Si, Cu, Mg, etc., which form a eutectic with aluminum is melted and then solidified at one end of the molten body, an aluminum fraction of high purity instantaneously separates out at the smooth interface between the liquid phase and the solid phase of the aluminum. Since the impurities are released into the liquid phase at the liquid-solid interface and become thereby concentrated, solidification thereafter proceeds through the growth of dentrites at the interface. The impurities released at the interface form crystals as such, or form eutectic crystals of several microns, between the dendrites or between the branches of dendrites. Accordingly such impure aluminum can be purified effectively by separating primary crystals or a pro-eutectic fraction of aluminum only from the aluminum in a molten state. U.S. Pat. No. 3,311,547, No. 3,671,229, No. 3,163,895 disclose processes for purifying aluminum by utilizing this procedure. With the process disclosed in U.S. Pat. No. 3,211,547, molten aluminum of low impurity is placed in a container opened at its upper end and maintained at a temperature higher than but close to the solidifying point of the melt. The melt is then cooled at its surface to form pro-eutectic aluminum. The pro-eutectic settles on the lower portion of the container, and the pro-eutectic deposit is compacted by suitable means to a block, which is separated from the mother liquor for recovery. Thus the purifying process requires the cumbersome procedure of compacting the whole deposit of the pro-eutectic with suitable means while accurately controlling the temperature of the melt. With the processs disclosed in U.S. Pat. No. 3,671,229, a cooled body is immersed in a melt of impure aluminum to form on the surface of the cooled body a pro-eutectic of aluminum, which is intermittently scraped off and caused to settle on the lower portion of the container. By suitable means, the pro-eutectic deposit is compacted to a block, which is finally collected. This process, like the foregoing process, also requires the procedure of periodically compacting the deposit and is therefore cumbersome. According to the process disclosed in U.S. Pat. No. 3,163,895, molten aluminum in a mold for continuously casting aluminum is agitated by a stirrer in the vicinity of the liquid-solid interface. Although capable of purifying the aluminum to some extent, this process involves a limitation on the purification efficiency.